I. Field of the Invention
The present invention relates to a hydrodynamic bearing device installed in a motor that rotationally drives a magnetic disk, optical disk, or other such recording disk, and more particularly relates to a hydrodynamic bearing device that is more compact and can be used in portable devices, and to a spindle motor and a recording and reproducing apparatus equipped with this hydrodynamic bearing device.
II. Background Information
A hydrodynamic bearing device in which the fluid pressure of oil or another such lubricating fluid interposed between a shaft and a sleeve is utilized to support the shaft and the sleeve so that they can rotate relative to one another has been proposed in the past as a bearing for spindle motors used in recording devices that rotationally drive a magnetic disk, optical disk, magneto optical disk or other such disk-shaped recording medium.
A series of tiny gaps are formed between the shaft and the sleeve, and a hydrodynamic groove formed in the circumferential direction of the rotational axis (radial hydrodynamic groove) and a hydrodynamic groove formed in the radial direction of the rotational axis (thrust hydrodynamic groove) are provided to the shaft or the sleeve or both. Oil is held as a lubricating fluid in these tiny gaps. One type of such hydrodynamic bearing devices has a structure in which a taper seal is formed at the end of the series of tiny gaps and exposed to the atmosphere, which is called a single-pocket structure.
With the single-pocket hydrodynamic bearing device disclosed in Japanese Laid-Open Patent Application 2005-308057, the shaft has a large-diameter flange portion at the bottom surface, and the flange portion is provided on a flange cover (cover plate). A thrust hydrodynamic groove is formed on the flange portion and/or the flange cover. The faces are flat everywhere except in the portion where the thrust hydrodynamic groove is formed.
With a conventional hydrodynamic bearing device structured like this, when the shaft begins to rotate, the oil is drawn into the radial and thrust bearing portions by the pumping pressure generated by the hydrodynamic grooves, and the fluid pressure rises within the hydrodynamic grooves. This results in a state in which the shaft and the sleeve are able to rotate relative to each other without touching.
In some cases, communicating holes are provided in the inner periphery of the flange portion, as seen in Japanese Laid-Open Patent Application 2003-28147, for example, in order to eliminate the pressure imbalance at the top and bottom of the thrust bearing portion during rotation.
With a conventional hydrodynamic bearing device, because the flange portion and the flange cover are made up of flat faces, and furthermore because the gap around the flange portion is relatively narrow (about 0.1 mm), vibration or impact due to a fall or the like tends to generate a portion of negative pressure at the center of the flange portion, so bubbles may be generated and as a result the lubricating fluid may leak out from the taper seal.
The mechanism of bubble generation will now be described. When the bearing device is stopped, the flange portion and the flange cover magnetically attract each other between the rotor magnet and the base, which is made of a magnetic material, and therefore come into contact with each other. If the bearing device should be dropped and subjected to an impact or vibration, so that the flange portion rises with the shaft, then the space between the sleeve and flange cover and the area around the flange portion increases by a volume corresponding to how far the main portion of the shaft has risen. When the shaft rises, the oil present on the upper surface of the flange portion in a stopped state attempts to move below the flange portion through the narrow gap between the bottom surface of the sleeve and the outer periphery of the flange portion. If the movement of the shaft is too sudden, however, the oil will be prevented by its own viscosity from sufficiently working its way below the flange portion. As a result, and coupled with the fact that the space between the sleeve and flange cover and the area around the flange portion increases by a volume corresponding to how far the main portion of the shaft has risen, a negative pressure portion is generated below the flange portion. Specifically, the center portion of the flange portion enters a vacuum state, and there is the risk that air or the like that has dissolved into the lubricating fluid after the work of filling the space with the lubricating fluid, for example, will create bubbles. Also, the surface of the oil exposed to the atmosphere at the taper seal rises according to how much oil did not work its way below the flange portion.
If a shaft that has risen should drop suddenly in a state in which bubbles have been generated, and before the bubbles have dissolved into the oil, the bubbles generated below the flange portion will be flattened out and rise, and depending on the volume of the generated bubbles, the surface of the oil that is exposed to the atmosphere at the taper seal will rise even more than in the initial stopped state. When this happens, there is the risk that the oil will leak out at the taper seal.
In particular, when communicating holes are provided passing through at the top and bottom of the sleeve for the purpose of pressure equalization, the oil is more apt to flow through the communicating holes than through the radial bearing portion, which means that bubbles are likely to enter the communicating holes. Bubbles that come through the communicating holes may cause the oil level to rise suddenly near the openings of the communicating holes, and the oil may leak out from the taper seal. If the oil leaks out, it can adversely affect the durability and performance of the hydrodynamic bearing device.
Even if communicating holes that communicate with the top and bottom of the flange portion are provided on the inner peripheral side of the flange portion as in Japanese Laid-Open Patent Application 2003-28147, if the distal end of the shaft is in the same plane as the bottom surface of the flange portion as shown in FIG. 19 of Japanese Laid-Open Patent Application 2003-28147, the bottom surface of the flange portion will tend to be under negative pressure in the event of an impact, the result being that bubbles are apt to be generated. Specifically, if the distal end of the shaft is in the same plane as the bottom surface of the flange portion, there is a smaller cross sectional area available for the oil to flow through the communicating holes from the upper surface of the flange portion to the bottom surface of the flange portion. As a result, if the shaft should move suddenly away from the flange cover, the flow resistance of the oil will increase, so the negative pressure on the bottom surface of the flange portion side will not be sufficiently eliminated, and bubbles will tend to be generated.
As shown in FIG. 5 of Japanese Laid-Open Patent Application 2003-28147, with a configuration in which the flange portion is fixed to the shaft with fixing screws, the screw head distal ends must be cross-shaped or star-shaped so that they can be turned with a screwdriver, and the shape is not in axial symmetry. Consequently, the oil is agitated by the screw heads during rotation, which makes it more likely that bubbles will be generated.